This invention relates to anti-inductive structures, and more particularly to electromagnetic interference (EMI) shielding utilized with miniaturized electronic equipment.
The packaging of modern electronic equipment such as radio communications equipment, radar, navigation equipment and the like requires the use of anti-inductive compartments for housing circuits generating radio-frequency energy. Such compartments generally include metallic or metalized walls around the periphery of a circuit emplaced on a planar element such as a printed circuit board. Three common types of EMI shielding structures utilize metal castings, stamped sheet metal, and metalized plastic for the walls. Each of these three types of manufacture requires expensive tooling and labor-intensive construction. The casting process limits minimum wall thickness to about 1.8 millimeters, and requires an expensive mold having a draft or taper therein to facilitate easy removal of the wall element from the mold. EMI shielding stamped from sheet metal requires the use of expensive blanking dies, forming equipment, and an assembly fixture for aligning and soldering together the parts which form the shielding structure. Further, extra process steps are often required to remove burrs from the stamped metal. Metalized plastic structures require relatively thick wall sections compared to stamped sheet metal, and also require an expensive mold for forming the plastic elements. The molded plastic parts must be metalized which requires additional process steps.
EMI shielding structures enclosing a discrete, circuit-bearing substrate such as a printed circuit board inside the structure are known, and are exemplified by U.S. Pat. Nos. 3,721,746 and 4,218,578. Circuit boards having a ground-plane element are known; see, e.g., U.S. Pat. No. 3,564,114. Separate structures for enclosing circuit-bearing substrates to provide EMI shielding therefor are expensive and require labor-intensive construction and assembly.